Document PCT/EP2011/071616 describes yeasts for producing alcohol in media containing at least one pentose, via the expression of the XI-XDH metabolic pathway. Said document describes for the first time the combination of an overexpression of xylose isomerase (XI) activity with an overexpression of xylitol dehydrogenase (XDH) activity and shows that the overexpression of xylitol dehydrogenase activity makes it possible to prevent the inhibition, by xylitol, of xylose isomerase activity. Document PCT/EP2011/071616 describes, for example, the yeast strain deposited at the CNCM [National Collection of Microorganism Cultures] on Oct. 5, 2011, under number I-4538.
The industrial production of alcohol by yeast from lignocellulosic hydrolyzates requires not only the genetic machinery for metabolizing pentoses, but also properties of resistance to the fermentation inhibitors present in these lignocellulosic hydrolyzates.
The fermentation inhibitors are produced during the pretreatment and the hydrolysis of the lignocellulosic biomass. Among the fermentation inhibitors are furaldehydes (furfural. HMF), phenolic compounds and organic acids (acetic acid, levulinic acid, formic acid).
Various means have been described for countering the effect of fermentation inhibitors, among which are detoxification of the fermentation medium, improvement of the fermentation process, or improvement of the resistance of yeasts to fermentation inhibitors.
The resistance of yeasts to fermentation inhibitors has, for example, been improved genetically, by directed evolution or by acclimatization (Almeida and Hahn-Hägerdal, International Sugar Journal, 2009. Vol. 111, No. 1323).
However, it appears to be difficult to provide an industrial yeast strain which gives yeasts which are both effective for alcohol production in a fermentation medium comprising at least one pentose, and resistant to at least one fermentation inhibitor.
There is thus a real need to provide novel yeast strains which are capable of metabolizing at least one pentose, for example xylose, even in the presence of at least one fermentation inhibitor, such as acetic acid, said yeast strains giving yeasts which are effective for producing alcohol, even in the presence of at least one fermentation inhibitor.